Implementing PageRank

[bkj]

PageRank, implemented from first principals. Should produce ~ the same results as the original Gunrock implementation, though convergence criteria may be different.

Suggestions -- none of these are major, but:

• would be nice to have a flag in operators::advance::execute to control whether frontiers are automatically swapped.
• could be nice to have a ForEachEdge that uses the advance load balancing to run a function on every edge
• likewise, could be nice to have a ForEachNode that runs a function on every node
• could be nice to have thrust wrappers that help the programmer manage streams -- otherwise, if I'm using thrust::copy_n or something, I have to be careful to use the right stream myself.
• could be nice to have an easy way to initialize the frontier w/ a sequence of nodes on device, rather than doing

    for (vertex_t v = 0; v < n_vertices; ++v)
     f->push_back(v);

Notes:

• Only works on unweighted graphs for now. Should work for weighted graphs now.
• May not handle "dangling" edges in directed graphs correctly at the moment Should treat dangling nodes the same was as networkx now.

[neoblizz]

• could be nice to have a ForEachEdge that uses the advance load balancing to run a function on every edge
• likewise, could be nice to have a ForEachNode that runs a function on every node

@porumbes very relevant for issue gunrock/gunrock#1001 (specifically the first bullet point).